Method for producing lithium solid state battery

ABSTRACT

A method for producing a lithium solid state battery having a solid electrolyte membrane with high Li ion conductivity, in which firm interface bonding is formed on both sides of the membrane, comprising steps of: a membrane-forming step of forming CSE1 not containing a binder, composed of a sulfide solid electrolyte material, on a cathode active material layer by an AD method and ASE1 not containing a binder, composed of a sulfide solid electrolyte material, on an anode active material layer by an AD method, and a pressing step of forming SE1 with the CSE1 and the ASE1 integrated by opposing and pressing the CSE1 and the ASE1, wherein the SE1 such that an interface between the CSE1 and the ASE1 disappeared is formed by improving denseness of the CSE1 and the ASE1 in the pressing step.

The present application is a continuation of application Ser. No.14/942,494 filed Nov. 16, 2015. The entire disclosure of the priorapplication is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method for producing a lithium solidstate battery having a solid electrolyte membrane with high Li ionconductivity, in which firm interface bonding is formed on both sides ofthe solid electrolyte membrane.

BACKGROUND ART

In recent years, in accordance with a rapid spread of informationrelevant apparatuses such as a personal computer, a video camera and aportable telephone, the development of a battery to be utilized as apower source thereof has been active. The development of a high-outputand high-capacity battery for an electric automobile or a hybridautomobile has been advanced also in the automobile industry. A lithiumion battery has the advantage that energy density is high among variouskinds of batteries.

Liquid electrolyte containing a flammable organic solvent is used for apresently commercialized lithium ion battery, so that a device forrestraining temperature rise during a short circuit and a device forpreventing the short circuit are necessary therefor. In contrast, alithium solid state battery all-solidified by replacing the liquidelectrolyte with a solid electrolyte layer intends the simplification ofthe safety device and is excellent in production cost and productivityfor the reason that the flammable organic solvent is not used in thebattery.

Various methods are known as a method for forming a solid electrolytelayer. In Patent Literature 1, a method for producing a sulfide-basedsolid state battery, in which sulfide-based solid electrolyte slurrycontaining a sulfide-based solid electrolyte, a binder and a fatty acidester is prepared and coated on one electrode to form an electrolytelayer and laminate the other electrode on the electrolyte layer, isdisclosed. Also, in Patent Literature 2, a blast method and an aerosoldeposition method are exemplified as a method for forming an electrolytewith a membrane thickness of 500 μm or less, composed substantially ofonly a lithium ion conductive solid substance.

Also, in Patent Literature 3, a blast method, an aerosol depositionmethod, a cold spray method, a sputtering method, a vapor growth methodand a thermal spraying method are exemplified as a method for forming asolid electrolyte layer. Also, in Patent Literature 4, a method forproducing a nonaqueous electrolyte battery, in which a cathode bodyhaving an amorphous cathode side solid electrolyte layer on a cathodeactive material and an anode body having an amorphous anode side solidelectrolyte layer on an anode active material layer are prepared andheat-treated while superposing the cathode side solid electrolyte layerand the anode side solid electrolyte layer so as to contact with eachother, and bonded by crystallizing the cathode side solid electrolytelayer and the anode side solid electrolyte layer, is disclosed.Incidentally, in Patent Literature 4, a vacuum deposition method, asputtering method, an ion plating method and a laser ablation method aredisclosed as a method for forming the cathode side solid electrolytelayer (PSE layer).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication (JP-A) No.2013-062228

Patent Literature 2: JP-A No. 2008-021424

Patent Literature 3: JP-A No. 2008-103203

Patent Literature 4: JP-A No. 2013-012416

SUMMARY OF INVENTION Technical Problem

For example, the sulfide-based solid electrolyte slurry in PatentLiterature 1 has a binder. The binder ordinarily does not have Li ionconductivity, so that the Li ion conductance of a solid electrolytemembrane having the binder becomes lower than the Li ion conductance ofthe sulfide-based solid electrolyte itself contained in the solidelectrolyte membrane.

On the other hand, in Patent Literatures 2 and 3, although specificexperimental results are not described, an aerosol deposition method (anAD method) is exemplified as a method for forming a solid electrolytemembrane. For example, in the case where a solid electrolyte membrane isformed on a cathode active material layer by an AD method, firminterface bonding may be formed between the solid electrolyte membraneand the cathode active material layer, but equal interface bonding maynot be formed between the solid electrolyte membrane and the anodeactive material layer. Thus, in the case of using an AD method, firminterface bonding is formed with difficulty on both sides of the solidelectrolyte membrane.

The present invention has been made in view of the actual circumstances,and the main object thereof is to provide a method for producing alithium solid state battery having a solid electrolyte membrane withhigh Li ion conductivity, in which firm interface bonding is formed onboth sides of the solid electrolyte membrane.

Solution to Problem

In order to solve the problems, the present invention provides a methodfor producing a lithium solid state battery, comprising steps of: amembrane-forming step of forming a solid electrolyte membrane (CSE1) notcontaining a binder, composed of a sulfide solid electrolyte material,on a cathode active material layer by an aerosol deposition method and asolid electrolyte membrane (ASE1) not containing a binder, composed of asulfide solid electrolyte material, on an anode active material layer byan aerosol deposition method; and a pressing step of forming a solidelectrolyte membrane (SE1) with the CSE1 and the ASE1 integrated byopposing and pressing the CSE1 and the ASE1, wherein the SE1 such thatan interface between the CSE1 and the ASE1 disappeared is formed byimproving denseness of the CSE1 and the ASE1 in the pressing step.

According to the present invention, the SE1 is formed from the CSE1 andthe ASE1 not containing a binder, so that a lithium solid state batteryhaving a solid electrolyte membrane with high Li ion conductivity may beobtained. Also, the CSE1 is formed on a cathode active material layer byan AD method and the ASE1 is formed on an anode active material layer byan AD method similarly to form the SE1 by bonding the CSE1 and the ASE1,so that a lithium solid state battery, in which firm interface bondingis formed on both sides of the SE1, may be obtained.

In the invention, a porosity of a cross section of the CSE1 and aporosity of a cross section of the ASE1 are each preferably within arange of 1% to 40%.

In the invention, a solid electrolyte membrane (CSE2) is preferablyformed on the cathode active material layer by a coating method beforeforming the CSE1.

In the invention, a solid electrolyte membrane (ASE2) is preferablyformed on the anode active material layer by a coating method beforeforming the ASE1.

In the invention, a membrane-forming treatment for forming a solidelectrolyte membrane (ASE3′) on the anode active material layer by anaerosol deposition method, and a pressing treatment for forming a solidelectrolyte membrane (ASE3) by pressing the ASE3′ are preferablyperformed before forming the ASE1.

In the invention, all solid electrolyte membranes existing between thecathode active material layer and the anode active material layer arepreferably formed by an aerosol deposition method.

Advantageous Effects of Invention

The present invention produces the effect such as to allow a lithiumsolid state battery having a solid electrolyte membrane with high Li ionconductivity, in which firm interface bonding is formed on both sides ofthe solid electrolyte membrane.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D are schematic cross-sectional views showing an example ofa method for producing a lithium solid state battery of the presentinvention;

FIG. 2 is a schematic view explaining an aerosol deposition method;

FIGS. 3A to 3D are schematic cross-sectional views each showing anotherexample of a method for producing a lithium solid state battery of thepresent invention;

FIGS. 4A to 4C are schematic cross-sectional views each showing anotherexample of a method for producing a lithium solid state battery of thepresent invention;

FIGS. 5A to 5D are schematic cross-sectional views each showing anotherexample of a method for producing a lithium solid state battery of thepresent invention;

FIGS. 6A to 6C are schematic cross-sectional views each showing anotherexample of a method for producing a lithium solid state battery of thepresent invention;

FIG. 7 is a schematic view showing a state during boost charge;

FIGS. 8A and 8B are schematic cross-sectional views each explaining adifference between a solid electrolyte membrane formed by a coatingmethod and a solid electrolyte membrane formed by an AD method;

FIGS. 9A and 9B are cross-sectional images of AD-SE before and afterpressing in Example 1;

FIG. 10 is a result of measuring membrane thickness and Li ionconductance of a solid electrolyte membrane (AD-SE after pressing) of anevaluation laminated body obtained in Example 1;

FIG. 11 is a result of measuring Li ion conductance of solid electrolytemembranes of evaluation laminated bodies obtained in Example 2 andComparative Example 1;

FIGS. 12A and 12B are cross-sectional images of AD-SE (before bonding)formed on an anode active material layer in Example 3;

FIGS. 13A and 13B are results of measuring output and discharge capacityof evaluation batteries obtained in Examples 4 and 5 and ComparativeExample 2;

FIGS. 14A and 14B are graphs each explaining a difference in inputperformance between the case of forming a solid electrolyte membrane byan AD method and the case of forming a solid electrolyte membrane by acoating method;

FIG. 15 is a cross-sectional image showing a difference between a solidelectrolyte membrane formed by an AD method and a solid electrolytemembrane formed by a coating method; and

FIG. 16 is a result of a compressive breaking test of a sulfide solidelectrolyte material.

DESCRIPTION OF EMBODIMENTS

A method for producing a lithium solid state battery of the presentinvention is hereinafter described in detail.

FIGS. 1A to 1D are schematic cross-sectional views showing an example ofa method for producing a lithium solid state battery of the presentinvention. In FIGS. 1A to 1D, first, a cathode active material layer 2is formed on a cathode current collector 1 to directly form a solidelectrolyte membrane 3 (CSE1) not containing a binder, composed of asulfide solid electrolyte material, on the cathode active material layer2 by an aerosol deposition method (an AD method) (FIG. 1A). Next, ananode active material layer 5 is formed on an anode current collector 4to directly form a solid electrolyte membrane 3 (ASE1) not containing abinder, composed of a sulfide solid electrolyte material, on the anodeactive material layer 5 by an AD method (FIG. 1B).

Here, FIG. 2 is a schematic view explaining an AD method. In FIG. 2, apedestal 12 is placed inside a chamber 11 and a substrate 13 is disposedon the pedestal 12. Also, the pressure inside the chamber 11 may becontrolled to an optional decompressed state by a rotary pump 14. On theother hand, raw material powder 16 is aerosolized inside an aerosolgenerator 17 by carrying-in gas supplied from a gas bomb 15. Inaddition, the aerosolized raw material powder is jetted from a nozzle 18disposed inside the chamber 11 toward the substrate 13. The accumulationwith plastic deformation of the particles is caused on the surface ofthe substrate 13 to form a solid electrolyte membrane.

Next, as shown in FIG. 1C, the CSE1 and the ASE1 are opposed andpressed. Thus, a solid electrolyte membrane (SE1) with the CSE1 and theASE1 integrated is formed (FIG. 1D). In particular, in the presentinvention, the SE1 such that an interface between the CSE1 and the ASE1disappeared is formed by improving denseness of the CSE1 and the ASE1 ina pressing step.

According to the present invention, the SE1 is formed from the CSE1 andthe ASE1 not containing a binder, so that a lithium solid state batteryhaving a solid electrolyte membrane with high Li ion conductivity may beobtained. Also, the CSE1 is formed on a cathode active material layer byan AD method and the ASE1 is formed on an anode active material layer byan AD method similarly to form the SE1 by bonding the CSE1 and the ASE1,so that a lithium solid state battery, in which firm interface bondingis formed on both sides of the SE1, may be obtained.

Also, in the present invention, the SE1 such that an interface betweenthe CSE1 and the ASE1 disappeared is formed, so that a solid electrolytemembrane with low interface resistance may be obtained. The reason whythe interface disappears is guessed to be that microstructural change iscaused at the interface between the CSE1 and the ASE1 in the process ofimproving denseness of the CSE1 and the ASE1 in a pressing step. Such aphenomenon is a new phenomenon which has not been known conventionally.Also, the case of forming a solid electrolyte membrane by an AD methodallows a dense membrane as compared with the case of forming a solidelectrolyte membrane by powder compacting; however, generally, a densermembrane must less cause a phenomenon such that the interfacedisappears. Yet, the inventors of the present invention have completedthe present invention by finding out that in a solid electrolytemembrane formed by an AD method, a peculiar phenomenon occurs, such thatmicrostructural change is caused through the pressing step despite thedense membrane and the interface disappears, and by applying the fact.

Also, the problem is that the sulfide-based solid electrolyte slurrydescribed in Patent Literature 1 has a binder, so that the Li ionconductance of a solid electrolyte membrane is low. In contrast, thepresent invention has the advantage that Li ion conductivity is high byreason of forming the SE1 from the CSE1 and the ASE1 not containing abinder. Also, in Patent Literatures 2 and 3, specific experimentalresults are not described, and neither description nor suggestion isoffered on a phenomenon such that the interface between the CSE1 and theASE1 disappears. Incidentally, in Patent Literature 2, a currentcollector not having Li ion conductivity is disposed between a cathodematerial and an anode material, so that a battery is not organized.Also, in Patent Literature 4, the cathode side solid electrolyte layerand the anode side solid electrolyte layer are bonded by crystallizing,but an idea of bonding by pressing is not described nor suggested.

The producing method for a lithium solid state battery of the presentinvention is hereinafter described in each step.

1. Membrane-Forming Step

The membrane-forming step in the present invention is a step of forminga solid electrolyte membrane (CSE1) not containing a binder, composed ofa sulfide solid electrolyte material, on a cathode active material layerby an AD method and a solid electrolyte membrane (ASE1) not containing abinder, composed of a sulfide solid electrolyte material, on an anodeactive material layer by an AD method.

(1) Formation of CSE1

In the present invention, a solid electrolyte membrane (CSE1) notcontaining a binder, composed of a sulfide solid electrolyte material,is formed on a cathode active material layer by an AD method. The CSE1may be formed directly on a cathode active material layer, or throughanother solid electrolyte membrane on a cathode active material layer.

(i) Raw Material Powder

In the present invention, particles of a sulfide solid electrolytematerial are used as raw material powder used for an AD method. Examplesof the sulfide solid electrolyte material include Li₂S—P₂S₅,Li₂S—P₂S₅—LiI, Li₂S—P₂S₅—LiCl, Li₂S—P₂S₅—LiBr, Li₂S—P₂S₅—Li₂O,Li₂S—P₂S₅—Li₂O—LiI, Li₂S—SiS₂, Li₂S—SiS₂—LiI, Li₂S—SiS₂—LiBr,Li₂S—SiS₂—LiCl, Li₂S—SiS₂—B₂S₃—LiI, Li₂S—SiS₂—P₂S₅—LiI, Li₂S—B₂S₃,Li₂S—P₂S₅—Z_(m)S_(n) (m and n are positive numbers, Z is any of Ge, Znand Ga.), Li₂S—GeS₂, Li₂S—SiS₂—Li₃PO₄, Li₂S—SiS₂—Li_(x)MO_(y) (x and yare positive numbers, M is any of P, Si, Ge, B, Al, Ga and In), andLi₁₀GeP₂S₁₂.

The sulfide solid electrolyte material preferably has Li, A (A is atleast one of P, Si, Ge, B and Al) and S. Above all, A is preferably atleast one of P, Si and Ge. Also, the sulfide solid electrolyte materialmay be amorphous, crystalline or glass ceramics.

Also, particles of the sulfide solid electrolyte material contain sulfuras an anionic component. Thus, the particles of the sulfide solidelectrolyte material have the property of being soft and easily deformedplastically as compared with particles of an oxide solid electrolytematerial. The hardness of the particles of the sulfide solid electrolytematerial may be evaluated through yield stress measurement by anano-indent method, for example. The yield stress of the particles ofthe sulfide solid electrolyte material is preferably within a range of300 MPa to 700 MPa, for example.

The average particle diameter D₅₀ of the raw material powder is notparticularly limited if the average particle diameter allows desiredCSE1, but is, for example, within a range of 100 nm to 10 μm,preferably, within a range of 500 nm to 5 μm.

(ii) AD Method

In the present invention, CSE1 is formed by an AD method. The AD methodallows a dense membrane with high adhesion properties by using‘normal-temperature shock solidification phenomenon’ (a phenomenon suchthat raw material powder solidifies with high density at normaltemperature without heating and only by applying mechanical shock). Inaddition, depending on the quality of the material of the membrane, theAD method has the advantage that the film-forming rate is several tensof times or more the rate of conventional thin-film-forming technology.Also, high pressure is applied to only so extremely limited region of asubstrate as to bring the advantage that the damage to the substrate issmall and interdiffusion due to heat is not caused. In particular, inthe case of using the sulfide solid electrolyte material as raw materialpowder, the sulfide solid electrolyte material collides with thesubstrate at high speed to deform plastically, and new surfaces of thesulfide solid electrolyte material bind with each other to allow a densemembrane without any gaps between particles. Also, the AD method has theadvantage that raw material powder collides with the substrate at sohigh speed that the membrane may be formed directly on the substrate byanchor effect.

In the AD method, collision rate at which raw material powder collideswith the substrate is not particularly limited if the collision rate isa rate such as to allow desired CSE1, but is preferably within a rangeof 100 m/s to 600 m/s, for example. Incidentally, collision rate may bemeasured by the method described in M. Lebedev et al., “Simpleself-selective method of velocity measurement for particles inimpact-based deposition”, J. Vac. Sci. Technol. A18(2), 563-566(2000).Specifically, the measurement is performed by using a collision ratemeasuring instrument, and the maximum rate V_(max) and the minimum rateV_(min) may be calculated by the following formula.

$\begin{matrix}{{V_{\max} = \frac{WL}{d_{1} + \frac{\delta}{2} + {L \cdot {\sin (\alpha)}}}}{V_{\min} = \frac{WL}{d_{2} - \frac{\delta}{2} - {L \times {\sin (\alpha)}}}}{W = {r\; \omega \mspace{14mu} \left( {m\text{/}s} \right)}}{\omega = {2{\pi/T}}}} & \left\lbrack {{Mathematical}\mspace{14mu} {Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the formula, “r” is a radius of rotation of a nozzle, T is arotational period of a nozzle, δ is a slit width, L is a distancebetween a slit and a substrate, α is an angle of divergence of aerosol,d₁ is a location of powder which reaches a substrate earliest through aslit, and d₂ is a location of powder which reaches a substrate latestthrough a slit.

The in-chamber pressure during membrane formation by the AD method isnot particularly limited but is, for example, 1 Pa or more, preferably,10 Pa or more. On the other hand, the in-chamber pressure is, forexample, 50 kPa or less, preferably, 1 kPa or less.

Kinds of carrier gas in the AD method are not particularly limited butexamples thereof include inert gases such as helium (He), argon (Ar) andnitrogen (N₂), and dry air. Also, the gas flow rate of carrier gas isnot particularly limited if the gas flow rate is a flow rate such as toallow desired aerosol to be maintained, but is preferably within a rangeof 1 L/min. to 20 L/min. with respect to a 500-mL aerosol vessel, forexample.

(iii) CSE1

The CSE1 obtained by the AD method is a solid electrolyte membrane notcontaining a binder, composed of a sulfide solid electrolyte material.The CSE1 is preferably composed of only a sulfide solid electrolytematerial. Also, the CSE1 is a solid electrolyte membrane subject tointerface bonding to the after-mentioned ASE1, and ordinarily has pores.These pores are pores such that an interface between the CSE1 and theASE1 disappears by pressing.

The degree of pores of the CSE1 may be evaluated by observing across-sectional image of the CSE1, for example. The porosity of a crosssection of the CSE1 may be, for example, 1% or more, 3% or more, 5% ormore, or 7% or more. On the other hand, the porosity of a cross sectionof the CSE1 may be, for example, 40% or less, 30% or less, 20% or less,or 15% or less.

The membrane thickness of the CSE1 (the membrane thickness beforepressing) becomes ordinarily larger than the average particle diameterof raw material powder, and is, for example, within a range of 100 nm to50 μm, preferably, within a range of 1 μm to 25 μm.

(iv) Cathode Active Material Layer

A cathode active material layer in the present invention contains atleast a cathode active material. In addition, the cathode activematerial layer may contain at least one of a solid electrolyte material,a conductive material and a binder. Kinds of the cathode active materialare not particularly limited but examples thereof include an oxideactive material. Examples of the oxide active material include rock saltbed type active materials such as LiCoO₂, LiMnO₂, LiNiO₂, LiVO₂ andLiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, spinel type active materials such asLiMn₂O₄ and Li(Ni_(0.5)Mn_(1.5))O₄, and olivine type active materialssuch as LiFePO₄, LiMnPO₄, LiNiPO₄ and LiCuPO₄.

Examples of the solid electrolyte material include the sulfide solidelectrolyte material described above. The sulfide solid electrolytematerial contained in the cathode active material layer and the sulfidesolid electrolyte material contained in the solid electrolyte membranemay be the same. Examples of the conductive material include carbonmaterials such as acetylene black, Ketjen Black, VGCF and graphite.Examples of the binder include a fluorine-containing binder such aspolyvinylidene fluoride (PVDF).

The cathode active material layer in the present invention may be amixture layer containing particles of the cathode active material, athin membrane layer of the cathode active material, or a sintered bodylayer of the cathode active material. A method for forming the cathodeactive material layer is not particularly limited but examples thereofinclude a coating method. In a coating method, ordinarily, slurry inwhich at least the cathode active material is dispersed into adispersion medium is applied on a substrate (such as a cathode currentcollector) and dried to thereby form the cathode active material layer.Also, the obtained cathode active material layer is preferably pressed.The reason therefor is to allow denseness of the cathode active materiallayer to be improved. In particular, in the case where the slurrycontains the sulfide solid electrolyte material, the cathode activematerial layer is preferably pressed by pressure (such as a pressure of2 ton/cm² or more) such that the sulfide solid electrolyte material isplastically deformed. The thickness of the cathode active material layeris preferably within a range of 0.1 μm to 500 μm, for example.

(2) Formation of ASE1

In the present invention, a solid electrolyte membrane (ASE1) notcontaining a binder, composed of a sulfide solid electrolyte material,is formed on an anode active material layer by an AD method. The ASE1may be formed directly on an anode active material layer, or throughanother solid electrolyte membrane on an anode active material layer.The conditions of the AD method and the membrane obtained by the ADmethod are the same as the contents described above; therefore, thedescription herein is omitted. Also, the sulfide solid electrolytematerial of the ASE1 and the CSE1 may be the same or different; theformer is preferable.

An anode active material layer in the present invention contains atleast an anode active material. In addition, the anode active materiallayer may contain at least one of a solid electrolyte material, aconductive material and a binder. Kinds of the anode active material arenot particularly limited but examples thereof include a carbon activematerial such as mesocarbon microbeads (MCMB), high orientation propertygraphite (HOPG), hard carbon and soft carbon, a metal active materialsuch as In, Al, Si and Sn, and an oxide active material such asLi₄Ti₅O₁₂.

Examples of the solid electrolyte material include the sulfide solidelectrolyte material described above. The sulfide solid electrolytematerial contained in the anode active material layer and the sulfidesolid electrolyte material contained in the solid electrolyte membranemay be the same. The conductive material and the binder are the same asthe contents described above.

The anode active material layer in the present invention may be amixture layer containing particles of the anode active material, a thinmembrane layer of the anode active material, or a sintered body layer ofthe anode active material. A method for forming the anode activematerial layer is not particularly limited but examples thereof includea coating method. In a coating method, ordinarily, slurry in which atleast the anode active material is dispersed into a dispersion medium isapplied on a substrate (such as an anode current collector) and dried tothereby form the anode active material layer. Also, the obtained anodeactive material layer is preferably pressed. The reason therefor is toallow denseness of the anode active material layer to be improved. Inparticular, in the case where the slurry contains the sulfide solidelectrolyte material, the anode active material layer is preferablypressed by pressure (such as a pressure of 2 ton/cm² or more) such thatthe sulfide solid electrolyte material is plastically deformed. Thethickness of the anode active material layer is preferably within arange of 0.1 μm to 500 μm, for example.

(3) Formation of CSE2 and ASE2

In the present invention, a solid electrolyte membrane formed by an ADmethod and a solid electrolyte membrane formed by a coating method maybe used in combination. The formation of a solid electrolyte membrane bya coating method allows mass productivity to be improved. Specifically,a solid electrolyte membrane (CSE2) may be formed on a cathode activematerial layer by a coating method before forming CSE1. Also, a solidelectrolyte membrane (ASE2) may be formed on an anode active materiallayer by a coating method before forming ASE1.

For example, in FIGS. 3A to 3D, a member, in which an anode activematerial layer 5 is formed on an anode current collector 4, and a memberhaving a solid electrolyte membrane 3 (ASE2) formed on a substrate 21 bya coating method are prepared (FIG. 3A). Next, the anode active materiallayer 5 and the ASE2 are pressed while opposed (FIG. 3B). Thus,denseness of the anode active material layer 5 and the ASE2 is improved,and simultaneously the ASE2 is transferred onto the anode activematerial layer 5 (FIG. 3C). Thereafter, ASE1 is formed on the ASE2 by anAD method (FIG. 3D). Incidentally, in FIGS. 3A to 3D, the ASE2 is formedon the anode active material layer 5 by transference, and the ASE2 maybe formed on the anode active material layer 5 by a coating method.

The sulfide solid electrolyte material of the CSE1 and the CSE2 may bethe same or different; the former is preferable. Similarly, the sulfidesolid electrolyte material of the ASE1 and the ASE2 may be the same ordifferent; the former is preferable. Also, the CSE2 and the ASE2 maycontain a binder or not contain a binder. The membrane thickness of theCSE2 or the ASE2 (the membrane thickness before pressing) is notparticularly limited but is within a range of 3 μm to 50 μm, forexample.

In a coating method, ordinarily, slurry in which the sulfide solidelectrolyte material is dispersed into a dispersion medium is applied ona substrate and dried to thereby form the solid electrolyte membrane(the CSE2 or the ASE2). Also, the obtained solid electrolyte membrane ispreferably pressed. The reason therefor is to allow denseness of thesolid electrolyte membrane to be improved. Specifically, the solidelectrolyte membrane is preferably pressed by pressure (such as apressure of 2 ton/cm² or more) such that the sulfide solid electrolytematerial is plastically deformed.

The present invention may offer a combination such that a member on theanode side has the ASE2 and a member on the cathode side does not havethe CSE2 as shown in FIG. 4A, a combination such that a member on thecathode side has the CSE2 and a member on the anode side does not havethe ASE2 as shown in FIG. 4B, or a combination such that a member on thecathode side has the CSE2 and a member on the anode side has the ASE2 asshown in FIG. 4C.

(4) Formation of CSE3 and ASE3

In the present invention, a solid electrolyte membrane obtained bypressing a membrane formed by an AD method may be used. The pressing ofa membrane formed by an AD method allows a denser and firmer solidelectrolyte membrane. Specifically, a membrane-forming treatment forforming a solid electrolyte membrane (CSE3′) on the cathode activematerial layer by an AD method before forming the CSE1, and a pressingtreatment for forming a solid electrolyte membrane (CSE3) by pressingthe CSE3′ may be performed. Similarly, a membrane-forming treatment forforming a solid electrolyte membrane (ASE3′) on the anode activematerial layer by an AD method before forming the ASE1, and a pressingtreatment for forming a solid electrolyte membrane (ASE3) by pressingthe ASE3′ may be performed.

For example, in FIGS. 5A to 5D, an anode active material layer 5 isformed on an anode current collector 4, and a solid electrolyte membrane3 (ASE3′) not containing a binder, composed of a sulfide solidelectrolyte material, is formed directly on the anode active materiallayer 5 by an AD method (FIG. 5A). Next, the ASE3′ is pressed (FIG. 5B).Thus, the sulfide solid electrolyte material contained in the ASE3′flows so as to fill up pores to allow a denser and firmer solidelectrolyte membrane (ASE3) (FIG. 5C). Thereafter, ASE1 is formed on theASE3 by an AD method (FIG. 5D). Thus, the ASE3 (or the CSE3) becomes sofirm a solid electrolyte membrane as to allow the solid electrolytemembrane to be restrained from cracking.

In particular, from the viewpoint of preventing a short circuit, thearea of the solid electrolyte membrane and the anode active materiallayer is occasionally made larger than the area of the cathode activematerial layer. In that case, stress due to the pressing concentrates onthe ends of the cathode active material layer, and the solid electrolytemembrane cracks to bring a possibility that the cathode active materiallayer and the anode active material layer contact and short-circuit. Asshown in FIGS. 5A to 5D, the placement of the firm solid electrolytemembrane (ASE3) in a member on the anode side allows the solidelectrolyte membrane 3 to be restrained from cracking due to stressconcentration.

The sulfide solid electrolyte material of the CSE1 and the CSE3 may bethe same or different; the former is preferable. Similarly, the sulfidesolid electrolyte material of the ASE1 and the ASE3 may be the same ordifferent; the former is preferable. The membrane thickness of the CSE3or the ASE3 (the membrane thickness before pressing) is not particularlylimited but is, for example, approximately the same as a numerical valuerange of the membrane thickness of the CSE1 described above.

The conditions of the AD method for forming the CSE3 or the ASE3 are notparticularly limited but may be equalized to the conditions of the CSE1described above. After forming the membrane by the AD method, the solidelectrolyte membrane is preferably pressed by pressure (such as apressure of 2 ton/cm² or more) such that the sulfide solid electrolytematerial is plastically deformed.

The present invention may offer a combination such that a member on theanode side has the ASE3 and a member on the cathode side does not havethe CSE3 as shown in FIG. 6A, a combination such that a member on thecathode side has the CSE3 and a member on the anode side does not havethe ASE3 as shown in FIG. 6B, or a combination such that a member on thecathode side has the CSE3 and a member on the anode side has the ASE3 asshown in FIG. 6C.

In the present invention, both the CSE2 and the CSE3 may be formedbetween the cathode active material layer and the CSE1 The order of theCSE2 and the CSE3 is not particularly limited but may be the order ofthe cathode active material layer, the CSE2, the CSE3 and the CSE1, orthe order of the cathode active material layer, the CSE3, the CSE2 andthe CSE1. This point is the same with regard to ASE.

Also, in the present invention, all solid electrolyte membranes existingbetween the cathode active material layer and the anode active materiallayer are preferably formed by the AD method. The reason therefor is toallow a lithium battery with favorable input characteristics. Examplesof the solid electrolyte membranes formed by the AD method include theCSE1, the CSE3, the ASE1 and the ASE3 described above.

2. Pressing Step

The pressing step in the present invention is a step of forming a solidelectrolyte membrane (SE1) with the CSE1 and the ASE1 integrated byopposing and pressing the CSE1 and the ASE1. Also, the SE1 such that aninterface between the CSE1 and the ASE1 disappeared is formed byimproving denseness of the CSE1 and the ASE1 in the pressing step.

In the present invention, ‘the interface between the CSE1 and the ASE1disappeared’ signifies, in cross-sectional observation of the SE1, thecase where the interface between the CSE1 and the ASE1 may not beconfirmed at all, and the case where the interface between the CSE1 andthe ASE1 may be confirmed slightly (for example, 5% or less with respectto the total length of the interface).

The pressure applied in the pressing step is not particularly limited ifthe pressure is a pressure such as to form the SE1 such that theinterface between the CSE1 and the ASE1 disappeared, but varies withdenseness of the CSE1 and the ASE1. The pressure is, for example, 1.5ton/cm² or more, preferably 2 ton/cm² or more, more preferably 4 ton/cm²or more. On the other hand, the pressure is, for example, preferably 10ton/cm² or less. Also, the time for applying the pressure is notparticularly limited but may be a time such as to allow the SE1, suchthat the interface between the CSE1 and the ASE1 disappeared, to beformed.

Also, in the pressing step, denseness of the CSE1 and the ASE1 isimproved by pressing to make the interface between the CSE1 and the ASE1disappear. Thus, in the pressing step, heating is not necessarybasically and pressing may be performed at room temperature; however,low-temperature heating may be performed as required. Examples of thelow-temperature heating include heating at temperature (for example,100° C. or less) less than crystallization temperature of the sulfidesolid electrolyte material.

Also, the SE1 is a solid electrolyte membrane with the CSE1 and the ASE1integrated. The membrane thickness of the SE1 (after pressing) is notparticularly limited but is, for example, within a range of 1 μm to 50μm, preferably, within a range of 5 μm to 20 μm.

3. Lithium Solid State Battery

In the present invention, the formation of the solid electrolytemembrane by the AD method allows a lithium solid state battery suitablefor boost charge. In the case of performing boost charge, as shown inFIG. 7, Li ions are not inserted into an anode active material 51 closeto an anode current collector 4, and a battery reaction is caused on thesurface of a solid electrolyte membrane 3 side of an anode activematerial layer 5. As a result, surface potential of the solidelectrolyte membrane 3 side of the anode active material layer 5decreases and Li precipitates.

Here, as shown in FIG. 8A, grain boundary of particles 31 of the sulfidesolid electrolyte material exists in the solid electrolyte membrane 3formed by a coating method. In addition, ordinarily, a binder (not shownin the drawing) for binding the particles 31 of the sulfide solidelectrolyte material is contained. Thus, a path for Li precipitated inthe anode active material layer 5 to reach a cathode active materiallayer (not shown in the drawing) exists in the solid electrolytemembrane 3 formed by a coating method. In contrast, as shown in FIG. 8B,the solid electrolyte membrane 3 formed by the AD method is so dense amembrane such that the sulfide solid electrolyte material is plasticallydeformed that the grain boundary scarcely exists. In addition,ordinarily, a binder is not contained. Thus, a path for Li to reach acathode active material layer (not shown in the drawing) existsextremely less in the solid electrolyte membrane 3 formed by the ADmethod even though Li is precipitated in the anode active material layer5. In this manner, the formation of the solid electrolyte membrane bythe AD method allows a lithium solid state battery suitable for boostcharge.

The lithium solid state battery obtained by the present inventioncomprises at least the cathode active material layer, the anode activematerial layer and the solid electrolyte membrane, ordinarily furthercomprising a cathode current collector for collecting the cathode activematerial layer and an anode current collector for collecting the anodeactive material layer. Examples of a material for the cathode currentcollector include SUS, aluminum, nickel, iron, titanium and carbon. Onthe other hand, examples of a material for the anode current collectorinclude SUS, copper, nickel and carbon. Also, the lithium solid statebattery obtained by the present invention may be a primary battery or asecondary battery, preferably a secondary battery. The reason thereforis to be useful as a car-mounted battery, for example. Examples of theshape of the lithium solid state battery include a coin shape, alaminate shape, a cylindrical shape and a rectangular shape.

Incidentally, the present invention is not limited to the embodiments.The embodiments are exemplification, and any is included in thetechnical scope of the present invention if it has substantially thesame constitution as the technical idea described in the claim of thepresent invention and offers similar operation and effect thereto.

EXAMPLES

The present invention is described more specifically while showingexamples hereinafter.

Example 1

First, a sulfide solid electrolyte material having Li₂S—P₂S₅ as the maincomponent was prepared as raw material powder. A solid electrolytemembrane (AD-SE) was formed on a carbon-coated substrate by an AD methodwith the use of this raw material powder. The film-forming conditionsare as follows.

<Film-Forming Conditions>

-   -   Average particle diameter D₅₀ of raw material powder 0.8 μm    -   Temperature normal temperature    -   Pressure in chamber 600 Pa    -   Gas He    -   Gas flow rate 20 L/min. (assist gas 19 L/min.)    -   Aerosol vessel 500 mL    -   Scan rate 10 mm/sec.    -   Distance between substrate nozzles 20 mm

When impact speed of the particles was calculated, V_(min) was 173 m/sand V_(max) was 505 m/s.

Thus, a member in which the solid electrolyte membrane (AD-SE) wasformed on the carbon coat was produced.

Incidentally, this AD-SE corresponds to CSE1 or ASE1. Next, two of themembers were pressed at a pressure of 1.5 ton/cm², 2.9 ton/cm², 4.3ton/cm² and 7.3 ton/cm² while disposing the AD-SE so as to oppose. Thus,a laminated body for evaluation was obtained.

[Evaluations]

(SEM Observation)

The cross section of the AD-SE before and after pressing was observed byusing a scanning electron microscope (SEM). The results are shown inFIGS. 9A and 9B. As shown in FIG. 9A, pores existed in the AD-SE beforepressing; however, as shown in FIG. 9B, pores scarcely existed in theAD-SE after pressing and the interface of the AD-SE disappeared.

(Membrane Thickness Measurement and Li Ion Conductance Measurement ofSolid Electrolyte Membrane)

Membrane thickness measurement and Li ion conductance measurement wereperformed for the solid electrolyte membrane (AD-SE) formed on acarbon-coated substrate. Incidentally, the solid electrolyte membrane(AD-SE) was pressed at the pressure described above to make theproduction conditions of the laminated body for evaluation correspond.Also, Li ion conductance was measured by an alternating currentimpedance method. The results are shown in FIG. 10. As shown in FIG. 10,when the pressure during pressing was increased, the membrane thicknessof the solid electrolyte membrane was decreased, so that it wassuggested that the increase of the pressure during pressing improved thedensity of the solid electrolyte membrane. Thus, it was guessed that thepressing of the solid electrolyte membrane caused microstructural changein the solid electrolyte membrane and the interface between the CSE1 andthe ASE1 disappeared. Also, a correlation was observed between thedecrease of the membrane thickness of the solid electrolyte membrane andthe improvement of Li ion conductance of the solid electrolyte membrane.Also, the membrane thickness of the AD-SE before pressing was 30 μm andthe solid electrolyte membrane converged on 21 μm by pressing, so thatthe membrane thickness decreasing rate was 30%. The membrane thicknessdecreasing rate in the present invention is preferably within a range of3% to 30%, for example.

Example 2

A solid electrolyte membrane (AD-SE) was formed on a carbon-coatedsubstrate on the film-forming conditions which were the same asExample 1. The membrane thickness of the AD-SE was determined atapproximately 400 μm.

Comparative Example 1

The raw material powder used in Example 2 and butylene rubber (BR) as abinder were dispersed into anhydrous heptane so that the ratio of thebinder was 1% by weight to obtain slurry. The obtained slurry was coatedon a substrate and pressed at a pressure of 1.5 ton/cm² or more tothereby obtain a solid electrolyte membrane.

[Evaluations]

(Li Ion Conductance Measurement)

Li ion conductance of the solid electrolyte membranes obtained inExample 2 and Comparative Example 1 was measured by an alternatingcurrent impedance method. The results are shown in FIG. 11. As shown inFIG. 11, in Example 2 not containing a binder, Li ion conductanceimproved by approximately 25% as compared with Comparative Example 1containing a binder. In addition, in Example 2, Li ion conductance equalto the raw material powder was obtained.

Example 3

First, natural graphite carbon as an anode active material, a sulfidesolid electrolyte material (average particle diameter D₅₀=0.8 μm) havingLi₂S—P₂S₅ as the main component, and PVDF as a binder were dispersedinto dehydrated butyl butyrate to obtain slurry. The obtained slurry wascoated on an anode current collector and pressed at a pressure of 2ton/cm² or more to thereby obtain an anode active material layer. Next,a sulfide solid electrolyte material having Li₂S—P₂S₅ as the maincomponent was prepared as raw material powder. A solid electrolytemembrane (AD-SE) was formed on the anode active material on thefilm-forming conditions, which were the same as Example 1, by an ADmethod with the use of this raw material powder.

Thus, a member in which the solid electrolyte membrane (AD-SE) wasformed on the anode active material layer was produced. Thereafter, themember was pressed at a pressure of 1.5 ton/cm² or more to thereby formthe pressed AD-SE. Incidentally, this AD-SE corresponds to ASE3.Thereafter, a solid electrolyte membrane (AD-SE) was formed byperforming an AD method again on the same conditions as the above. ThisAD-SE corresponds to ASE1. Incidentally, the thickness of the ASE1 aimedfor 7 μm.

On the other hand, LiNi_(1/3)Mn_(1/3)CO_(1/3)O₂ as a cathode activematerial, a sulfide solid electrolyte material (average particlediameter D₅₀=0.8 μm) having Li₂S—P₂S₅ as the main component, and PVDF asa binder were dispersed into dehydrated butyl butyrate to obtain slurry.The obtained slurry was coated on a cathode current collector andpressed at a pressure of 2 ton/cm² or more to thereby obtain a cathodeactive material layer. Thereafter, a solid electrolyte membrane (AD-SE)was formed on the cathode active material layer by performing an ADmethod on the same conditions as the above. This AD-SE corresponds toCSE1. Incidentally, the thickness of the CSE1 aimed for 3 μm. Lastly,the CSE1 and the ASE1 were pressed at a pressure of 1.5 ton/cm² or morewhile disposed so as to oppose. Thus, an evaluation battery wasobtained.

[Evaluations]

(SEM Observation)

The cross section of the AD-SE (before bonding) formed on the anodeactive material layer was observed by using a scanning electronmicroscope (SEM). The results are shown in FIGS. 12A and 12B.Incidentally, FIG. 12B is a magnified view of the AD-SE in FIG. 12A. Asshown in FIG. 12A, when the pressed AD-SE and the unpressed AD-SE werecompared, it may be confirmed that the density improved. It may beconfirmed from this fact that the pressing of the solid electrolytemembrane caused microstructural change in the solid electrolytemembrane. Image analysis was performed for the AD-SE shown in FIG. 12Band the porosity of a cross section of the solid electrolyte membranemeasured 9%. Also, in the obtained evaluation battery, the interfacebetween the CSE1 and the ASE1 disappeared. On the other hand, thepressed AD-SE was pressed at a pressure of 1.5 ton/cm² or more whileopposed; however, the interface did not disappear.

Example 4

First, natural graphite carbon as an anode active material, a sulfidesolid electrolyte material (average particle diameter D₅₀=0.8 μm) havingLi₂S—P₂S₅ as the main component, and PVDF as a binder were dispersedinto dehydrated butyl butyrate to obtain slurry. The obtained slurry wascoated on an anode current collector and pressed at a pressure of 2ton/cm² or more to thereby obtain an anode active material layer. Next,a sulfide solid electrolyte material having Li₂S—P₂S₅ as the maincomponent was prepared as raw material powder. A solid electrolytemembrane (AD-SE) was formed on the anode active material on thefilm-forming conditions, which were the same as Example 1, by an ADmethod with the use of this raw material powder.

Thus, a member in which the solid electrolyte membrane (AD-SE) wasformed on the anode active material layer was produced. This AD-SEcorresponds to ASE1. Incidentally, the thickness of the ASE1 aimed for 7μm.

On the other hand, LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂ as a cathode activematerial, a sulfide solid electrolyte material (average particlediameter D₅₀=0.8 μm) having Li₂S—P₂S₅ as the main component, and PVDF asa binder were dispersed into dehydrated butyl butyrate to obtain slurry.The obtained slurry was coated on a cathode current collector andpressed at a pressure of 2 ton/cm² or more to thereby obtain a cathodeactive material layer. Thereafter, a solid electrolyte membrane (AD-SE)was formed on the cathode active material layer by performing an ADmethod on the same conditions as the above. This AD-SE corresponds toCSE1. Incidentally, the thickness of the CSE1 aimed for 3 μm. Lastly,the CSE1 and the ASE1 were pressed at a pressure of 1.5 ton/cm² or morewhile disposed so as to oppose. Thus, an evaluation battery wasobtained. In the obtained evaluation battery, the interface between theCSE1 and the ASE1 disappeared.

Example 5

Before forming a solid electrolyte membrane corresponding to the ASE1and the CSE1 in Example 4, solid electrolyte membranes (ASE2, CSE2) wereformed on an anode active material layer and a cathode active materiallayer respectively by a coating method. Specifically, the raw materialpowder used in Example 4 and BR as a binder were dispersed intoanhydrous heptane so that the ratio of the binder was 1% by weight toobtain slurry. The obtained slurry was coated on a substrate and pressedat a pressure of 1.5 ton/cm² or more to thereby obtain a solidelectrolyte membrane. The thickness of the solid electrolyte membraneaimed for 5 An evaluation battery was obtained in the same manner asExample 4 except for forming the ASE2 and the CSE2, and aiming for 3 asthe thickness of the ASE1 and the CSE1. In the obtained evaluationbattery, the interface between the CSE1 and the ASE1 disappeared.

Comparative Example 2

First, an anode active material layer and a cathode active materiallayer were produced in the same manner as Example 4. Next, the rawmaterial powder used in Example 4 and BR as a binder were dispersed intoanhydrous heptane so that the ratio of the binder was 1% by weight toobtain slurry. The obtained slurry was coated on an aluminum foil anddried. Next, the dried surface was contacted with the anode activematerial layer and pressed at a pressure of 1.5 ton/cm² or more. Next,the aluminum foil was removed to form a solid electrolyte membrane onthe anode active material layer. Thereafter, the solid electrolytemembrane and the cathode active material layer were opposed and pressedat a pressure of 2 ton/cm² or more to thereby obtain an evaluationbattery.

[Evaluations]

(Output Measurement and Discharge Capacity Measurement)

The evaluation batteries obtained in Examples 4 and 5 and ComparativeExample 2 were constrained at 150 kgf to measure output and dischargecapacity. The measurement conditions of output are as follows. That isto say, the output was measured by performing CCCV charge at SOC 60%voltage and current value ⅓ C. and discharge at 60 mW/cm², andthereafter performing CCCV charge at SOC 60% voltage and current value ⅓C. and discharge repeatedly at 80 mW/cm², 100 mW/cm² and 120 mW/cm². Themeasurement conditions of discharge capacity are as follows. That is tosay, the discharge capacity was measured by performing CCCV charge atcut voltage 4.55 V and current value ⅓ C, halting for 10 minutes, andperforming CCCV discharge at cut voltage 3 V and current value ⅓ C up toend current 1/100 C. The results are shown in FIGS. 13A and 13B. Asshown in FIGS. 13A and 13B, in Example 4 (the case of forming the solidelectrolyte membrane by only an AD method) and Example 5 (the case offorming the solid electrolyte membrane by an AD method and a coatingmethod), as compared with Comparative Example 2 (the case of forming thesolid electrolyte membrane by only a coating method), the outputimproved and the capacity was equal.

Also, for reference, a difference in input performance between the caseof forming the solid electrolyte membrane by an AD method and the caseof forming the solid electrolyte membrane by a coating method is shownin FIGS. 14A and 14B. As shown in FIGS. 14A and 14B, in the case offorming the solid electrolyte membrane by an AD method, inputperformance improves greatly. The reason therefor is guessed to be thatthe solid electrolyte membrane formed by an AD method is so denser thanthe solid electrolyte membrane formed by a coating method as toeffectively restrain Li from precipitating and growing in the anodeactive material layer. Actually, as shown in FIG. 15, in the solidelectrolyte membrane formed by a coating method, the presence of a grainboundary may be confirmed over the whole region; however, in the solidelectrolyte membrane formed by an AD method, a grain boundary isscarcely present.

Reference Example

A compressive breaking test was performed for the raw material powder(the sulfide solid electrolyte material having Li₂S—P₂S₅ as the maincomponent) to calculate yield stress. First, the raw material powder wasdispersed into anhydrous heptane by using an ultrasonic homogenizer, andtaken out by a micropipet. Next, the taken solution was dropped on acopper stage by several μL and dried directly. Next, SEM observation wasperformed on the copper stage to record microstructure and location ofthe intended raw material powder. Next, the copper stage was moved to ananoindenter to evaluate mechanical characteristics.

The results are shown in FIG. 16. As shown in FIG. 16, the yield stressof the raw material powder was approximately 600 MPa. When the sameexperiment was performed plural times, the yield stress of the rawmaterial powder was within a range of 400 MPa to 650 MPa, which wassimilar to metal aluminum. Also, a compressive breaking curve exhibitedplastic deformation behavior similarly to metal aluminum.

REFERENCE SIGNS LIST

-   1 cathode current collector-   2 cathode active material layer-   3 solid electrolyte membrane-   4 anode current collector-   5 anode active material layer-   11 chamber-   12 pedestal-   13 substrate-   14 rotary pump-   15 gas bomb-   16 raw material powder-   17 aerosol generator-   18 nozzle

What is claimed is:
 1. A lithium solid-state battery comprising: acathode active material layer, a cathode-side solid electrolyte membranenot containing a binder, composed of a sulfide solid electrolytematerial, and formed on the cathode active material layer, an anodeactive material layer, and an anode-side solid electrolyte membrane notcontaining a binder, composed of a sulfide solid electrolyte material,and formed on the anode active material layer, wherein the cathode-sidesolid electrolyte membrane and the anode-side solid electrolyte membraneare opposed, the cathode-side solid electrolyte membrane and theanode-side solid electrolyte membrane have a disappeared interface whenobserved by a scanning electron microscope, and the cathode-side solidelectrolyte membrane and the anode-side solid electrolyte membrane havean interface with the length of 5% or less to the total length.
 2. Thelithium solid-state battery according to claim 1, wherein a totalmembrane thickness of the cathode-side solid electrolyte membrane andthe anode-side solid electrolyte membrane is within a range of 1 μm to50 μm.